Various approaches to removing ice from aircraft surfaces have been tried. In one approach, an airfoil surface shape is altered by utilization of parasitic elements. This particular approach is exemplified in U.S. Pat. Nos. 2,297,951 to Frank, 2,201,155 to Burgess and 2,872,987 to Gahagan. In the '951 patent to Frank, a flexible strip is mounted to a wing leading edge, and is flexed by a plurality of rollers in order to remove ice. The '155 patent discloses a flexible skin arranged over the leading edge of a wing such that the skin may be flexed by operation of ballrollers, friction links, pistons and the like. In the '987 patent a spar is provided in running arrangement through a wing. When the spar is flexed in a vertical direction, a plurality of vertical wing sections are rocked causing the wing to flex.
In the spar arrangement of the '987 patent, a high price is paid in structural efficiency since the spar, which carries wing loads, must be designed within a much smaller envelope than is the case with conventional stressed skin structures. In addition, the airfoil segments constitute additional weight and can carry none of the spanwise bending or axial load. The '951 and '155 patents also involve operation of devices and elements which provide little or no structural purpose but which are parasitic to the basic wing structure and covering. These parasitic elements are undesirable in that they represent large additional weight without performing primary structural functions.
Two systems intended to operate by altering or deforming wing surfaces are described in U.S. Pat. Nos. 2,037,626 to Hall and 2,135,119 to Wood. The '626 patent utilizes a mechanical device to modify the contour of the leading edge of an airfoil during flight primarily to control wing characteristics and give the wing a lift curve of desired shape, but also for the purpose of breaking up and removing ice. A longitudinal member mounted in the leading edge of the wing is projected or retracted to alter the wing leading edge contour. In the '119 patent, a plurality of slats or vibrators are mounted on the leading edge of a wing. The slats are vibrated relative to each other to prevent formation of or to break-up ice on the wing surface.
Tests have shown that ice deposited on airfoil surfaces in flight adheres strongly to the surface, even when such surfaces are of so-called "hydrophobic" materials (as, for example, polyethylene and teflon). Because of this effect it is seldom sufficient for the surface to be simply vibrated, as in the '119 patent, or generally deformed or altered, as in the '626 patent, in order to dislodge ice. An appreciable surface distortion must be introduced, of a type which produces high shearing stress at the interface between the ice and the airfoil surface sufficient to exceed the adhesion or bond strength. This can be accomplished by large deflections effected over a long span, or by small deflections effected over a short span, the significant quantity being the change in the local radius of curvature of the surface, as this is directly related to the magnitude of the shearing stress introduced into the ice-to-airfoil surface bond. As a result, these concepts cannot be implemented to practical advantage. On the one hand, large deflections require a flexible surface which is inefficient structurally for carrying the primary air loads and wing bending loads, and thus is heavy and unattractive. On the other, small local deflections may be introduced mechanically or by activators as described in the '119 patent into more rigid surfaces such as those desired for efficient wing construction, but these actuators must be numerous and powerful to achieve the required deflection over short spans, and thus will comprise undesirable additional weight.
Another approach to removing ice from airfoil surfaces is disclosed in U.S. Pat. No. 1,819,497 to Chisholm, which discloses the use of electrical energy to generate heat in the airfoil surface to raise the temperature and cause the ice to loosen and be blown off. Removal of ice through mechanical action (bond failure) which causes the bond strength between the ice and the airfoil to be exceeded, has been shown to be a more efficient ice removal system with respect to energy requirements than the heating system of the '497 patent. In addition, such heating systems are particularly less efficient at low temperatures, when energy requirements of simple heating systems become very large.
Somewhat more promising deicing systems are the electromagnetic inductive pulse deicing systems for deicing airfoil surfaces by bringing about localized elastic deformation of the surface of the airfoil. See for example, the various systems disclosed in U.S. Pat. No. 3,549,964 to Levin et al and British Patent Specification No. 505,433 to Goldschmidt. The '964 patent discloses means for transforming electric current pulses separated by time intervals into mechanical power pulses. The system includes wire loops connected to a current pulse source which are positioned in close proximity to the surface of the airfoil to be deiced. A primary current is produced in the wire loops while current is induced by electromagnetic induction flow through the airfoil surface. Pulses of mechanical force normal to the surface result from the interaction between the magnetic field of the loop primary current and the induced current in the airfoil surface, which pulses cause elastic deformations of the surface to remove ice therefrom. The British patent broadly discloses utilization of pulsating electromagnetic forces between conducting sheets of an airfoil. Alternating or pulsating electrical currents are caused to flow in parallel or opposite directions in the sheets, the electromagnetic interactions causing vibration and/or wavelike deformation of the outer sheet, which might for example make up the leading edge of an airfoil. Examples of electrical pulse distribution systems which may be utilized in conjunction with electromagnetic inductive pulse deicing systems may be found in U.S. Pat. Nos. 3,672,610 and 3,779,488 both to Levin, as well as, in a somewhat more primitive form, in the '433 British specification.
Both the '964 patent and the British patent generally describe wave-type deformation which emanates from the point or area of the mechanical impulse. The wave propagation from these types of systems has been found to be less than highly efficient. Moreover, the eddy currents which are induced by the device of the '964 patent and at least one embodiment of the British patent are either greatly restricted or effectively eliminated for various types of aircraft materials, as for example titanium, titanium alloys, laminates such as glass, graphite, and Kevlar fiber-reinforced epoxy matrix composites, and even to a large degree, steels, including the stainless alloys. In addition, the use of additional metal sheets in some of the embodiments of the British patent, which are separated from the primary wing structure and are positioned externally to it, adds highly undesirable weight to the overall structure.
An electromagnetic inductive pulse deicing system which was developed for use in cramped areas is disclosed in U.S. Pat. No. 3,809,341 to Levin et al. One embodiment of the '341 patent utilizes flat buses, of rectangular or trapezoidal section, two of which are arranged one beneath the other with the outside surface of one of them being located adjacent to the surface being deiced. Current flowing through the buses causes their relative displacement in a direction normal to the surface being cleaned, providing for an elastic deformation of the surface. Various embodiments of the '341 invention utilize flat buses, or other conductors, which mate with or lie in grooves or slots on the surface of the element being deiced. As in the case with the aforementioned British and '964 patents, this patent describes a mechanism which brings about a less than highly efficient electromagnetic action which emanates from the point or general area of the mechanical impulse. In addition, the special arrangements of conductors, spacers, stringers, supports and the like disclosed in the '341 patent render economical and efficient manufacturing procedures difficult to attain.
Finally, a broad reference to the utilization of an electromagnetic artificial muscle for de-icing purposes is suggested in U.S. Pat. No. 2,532,876 to Asche et al. The artificial muscle in the '876 patent operates as a result of the force created by a magnetic field on paramagnetic particles. No disclosure is provided as to the use of such a muscle in airfoil environments, and it would appear to be quite complex, if at all possible, to adapt a system of such muscles for use with an airfoil for deicing purposes.
Performance of prior art airfoil electromagnetic inductive pulse deicing systems is often limited by a number of factors, including but not limited to (1) the stresses produced in the airfoil skin, which establish a fatigue design condition, and (2) the magnitude of the deformation produced for a given size coil and electrical supply. For a given level of ice removal, a certain initial skin deflection is required. This deflection requires a large force input from the electro-impulse system and it also results in a high fatigue stress level at the nose of the airfoil leading edge.
An especially efficient, economic and desirable electromagnetic inductive pulse deicing mechanism utilizable with airfoil and other surfaces constructed of high electrical resistance materials, such as titanium, steel, or composites, is disclosed in my co-pending U.S. patent application Ser. No. 214,489, filed Dec. 9, 1980, for "Conductive Skin-Doubler Plate And Secondary Skin-Coil Electromagnetic Inductive Pulse Deicing Systems", assigned to the assignee herein. The application discloses a system which includes skin-doubler plates and/or skin-coils which are secured to or within an airfoil surface constructed of a high resistance material. The skin-doubler plates and coils are constructed of a low resistance material, and are positioned such that they are aligned with a source of impulse current. In this particular arrangement, eddy currents may be readily induced in the skin-doubler plates and coils so as to bring about elastic deformation of the skin surface, even though it primarily consists of a high resistance material.
In view of the problems associated with the aforementioned prior art approaches, the disclosures of which are hereby incorporated by reference herein, there exists a need to develop a highly efficient system for the removal of ice from airfoil and other surfaces.
From the foregoing, it can be seen that it is a primary object of the present invention to provide an electromagnetic pulse deicing system that is highly efficient in minimizing limitations as a result of stress-fatigue design conditions and magnitude of deformation for a given size coil and electrical supply.
Another object of the present invention is to utilize a unique coil arrangement in an airfoil electromagnetic pulse deicing system such that a deflection mode, involving torsion of the airfoil nose as well as unsymmetrical bulging of the airfoil skin panels, is produced. As a consequence, less force is required to obtain equivalent deflection for ice removal and lower stresses result at the nose.
It is yet another object of the present invention to provide an electromagnetic pulse deicing system for introducing an appreciable surface distortion, in the form of small deflections effected over a short span, in an airfoil surface which produces high shearing stress at the interface between the airfoil surface and ice which is adhered to the surface.